A study concerning the substrate conformational preferences displayed by cytidine deaminase (CDA) versus the mechanistically similar enzyme, adenosine deaminase (ADA), revealed important differences between the enzymes that were explained on the basis of sugar puckering and the role of the ribose oxygen in binding and catalysis (Nucleosides Nucleotides &amp;Nucleic Acids 2009, 28, 614-632). In a study using the most potent CDA inhibitor that our lab developed in the decade of the 80s, a diazepinone riboside, the ribose ring was replaced with conformationally locked pseudosugars built with a bicyclo[3.1.0]hexane template. Taking advantage of the stability of the carbocyclic moiety a very stable CDA inhibitor that almost equal in potency the parent diazepinone riboside was disovered (J. Org. Chem. 2009 in press).